1. Field of the Invention
This invention relates to an optical information input-output device which is a pickup optical system of an optical information recording apparatus or reproducing apparatus such as a video disc, a photomemory or a photomagnetic memory.
2. Description of the Prior Art
To record information on an optical information recording medium such as a video disc, a photomemory or a photomagnetic memory or to reproduce the information from the recording medium, there is required an optical system of the construction as shown, for example, in FIG. 1 of the accompanying drawings. That is, in such optical information input-output device of the prior art, a laser light emitted from a laser light source 1 is collimated by a collimater lens 2, passes through a polarizing beam splitter 3 and is converted from a linearly polarized light into a circularly polarized light by a quarter wavelength plate 4. This light is further reflected by a pivoting mirror 5 for tracking control and is condensed through an objective lens 6 onto the surface of an information recording disc 7 such as a video disc or a photomemory. The light reflected by the disc 7 travels back along the aforementioned route and is converted into a linearly polarized light by the quarter wavelength plate 4, and then is reflected in the polarizing beam splitter 3 toward a photodiode 8. Isolation of the light is accomplished by the polarizing beam splitter 3 and the quarter wavelength plate 4 so that the light reflected by the disc 7 is prevented from entering the light source 1 and is directed to the photodiode 8. In the photodiode 8, the information on the disc 7 can be read by the intensity of the light.
The pivoting mirror 5 has its pivotal movement mechanically controlled in the direction of arrow by a magnetic field formed by an electric current so that the light is oscillated radially of the disc 7. The vibration of the track resulting from the eccentricity, vibration or the like of the disc 7 is detected by some means and the pivoting mirror 5 is operated by this tracking error signal so that the spot of the laser light does not deviate from a predetermined track on the surface of the disc 7.
The objective lens 6 has the function of condensing the laser light into a spot of a diameter of the order of 1 .mu.m on the disc 7 and is mechanically movable perpendicularly to the disc 7 by a magnetic field formed by an electric current. Also, this objective lens 6 is controlled in accordance with focus error information so that the spot diameter always becomes smallest on the surface of the disc 7 for vibration of the disc 7 in the direction of the optical axis.
Thus, in the device of the prior art, various control means are mechanically accomplished, for example, by a current magnetic field, and this has led to a great number of parts and a great amount of consumed power and moreover, the responsiveness of the device has been limited.
Also, in the device of the prior art, as previously mentioned, a polarizing beam splitter and a quarter wavelength plate have been required for separating the laser light and the reflected light of the laser light from the disc, and this has led to the high cost of the device and has been disadvantageous in making the device compact.